1. Technical Field
The invention belongs to the field of electrical engineering/electronics and relates to an oscillator circuit, including a combination comprising two frequency-determining elements and an active electronic circuit, the frequency-determining elements being configured as one-port surface wave resonators having interdigital transducers.
Objects for which the application of the invention is possible and advantageous include components based on surface acoustic waves, such as oscillators and sensors, and particularly sensors for which the oscillator frequency temperature response can be adjusted.
2. State of the Art
Oscillator circuits are known, which include a combination comprising two parallel or series connected frequency-determining elements and an active electronic circuit having negative incremental resistance, or negative incremental conductance, wherein the frequency-determining elements are configured as one-port surface wave resonators having interdigital transducers, the synchronous frequency first-order temperature coefficients of the two one-port surface wave resonators having different algebraic signs, while the synchronous frequency second-order temperature coefficients of the two one-port surface wave resonators have the same algebraic signs.
In a particular configuration, the combination comprising two frequency-determining elements comprises two one-port surface wave resonators, the substrates of which are part of one and the same crystal section, but which use different propagation directions (DE 29 38 158 A1). The transducers of the one-port surface wave resonators are connected in parallel. The crystal section used is ST cut quartz. The substrate of the main resonator uses the X axis of quartz as the direction of propagation, while the propagation direction of the auxiliary resonator is aligned at 41° relative thereto. Accordingly, the first order temperature coefficient of the synchronous frequency is eliminated for the main resonator. In contrast, the first order temperature coefficient of the synchronous frequency of the auxiliary resonator is other than zero, Despite the different orders of the temperature coefficients, it is possible to achieve compensation of the second order temperature coefficient of the synchronous frequency of the main resonator. The first order temperature coefficient of the synchronous frequency of the auxiliary resonator required for the compensation of the second order temperature coefficient of the synchronous frequency of the main resonator is stated as a function of the second-order temperature coefficient to be compensated, the amplitude of the auxiliary resonator, and the propagation distance, which is equal for both resonators. This solution discloses no suggestion of an oscillator circuit for one- port surface wave resonators. However, it be assumed that the manner in which an oscillator circuit comprising one-port surface wave resonators can be configured is known.
A known solution in connection with remotely queried sensors is to combine two one-port surface wave resonators for temperature compensation, wherein the substrates of these resonators present different propagation directions of one and the same crystal section (A differential measurement SAW device for passive remote sensoring, W. Buff, M. Rusko, T. Vandahl, M. Goroll, and F. M ler, Proc. 1996 IEEE Ultrasonics Symposium, pgs. 343-346 [3]). It is a prerequisite for temperature compensation that these propagation directions have different phase velocities and nearly identical synchronous frequency temperature coefficients.
A previously proposed particular configuration for an oscillator circuit includes a combination comprising two frequency-determining elements and an active electronic circuit, the frequency-determining elements being configured as one-port surface wave resonators having interdigital transducers. The substrates of the one-port surface wave resonators are part of one and the same crystal section, but have different propagation directions (DE 10 2005 060 924.4). An inductance is connected in parallel with the transducer of each one-port surface wave resonator. Two circuits of this type are connected in series, the one-port surface wave resonators present in these circuits differing in the propagation directions thereof. The synchronous frequency first-order temperature coefficients of the two one-port surface wave resonators differ with respect to their algebraic signs. By suitably selecting the inductors and apertures of the one-port surface wave resonators, it is possible to compensate both the first-order and the second-order oscillator frequency temperature coefficients.
The proposed solution has the disadvantage that, at undesirable frequencies, the inductances present in the oscillator circuit can result in oscillation states for the oscillator, which are not stabilized by the one-port surface wave resonators with respect to the temperature sensitivity thereof.